1. Field of the Invention
The present invention relates to a wafer processing method and an equipment therefor, in particular, to a method for finishing the chamfered peripheral portion of a wafer and a finishing equipment therefor.
2. Description of Related Art
Conventionally, a wafer processing method comprising the steps of; chamfering a peripheral portion of a wafer for preventing the peripheral portion from being chipped off, lapping for making variation in thickness of wafers small, etching for removing a cracked layer and contaminated portions which were formed by abrasive grains sticking thereunto or the like, and mirror-polishing the chamfered peripheral portion and the main surface of the wafer, sequentially, is known. A method in which the order of the chamfering and lapping steps is reversed to that of the above-described method, is also known, as described in "Semiconductor Material Basic Engineering" published by Nikkan Kogyo Newspaper Publishing Company on Feb. 28, 1994. However, in the latter method, because the peripheral portion of the wafer remains edged when lapping, there are the danger of occurrence of the peripheral portion being chipped off during lapping and further the danger of the main surface of the wafer being scratched by broken silicon pieces or the like. Therefore, the method comprising a lapping step after a chamfering step, such as the former, is recently on the main stream.
There is another method as a modified example of the former, in which the chamfering step comprises a first grinding step for grinding to round and chamfer the peripheral portion of the wafer by using a grinding stone having large grain size (e.g., #800) and a second grinding step for grinding the chamfered portion by using a grinding stone having small grain size (e.g., #1500) just after the first grinding step. According to the method, the smoothness of the chamfered portion is a little degraded in a following etching step. However, because the smoothness thereof after the etching step is better than the case using only a grinding stone having large grain size, it is possible to easily carry out the work in a following polishing step for the chamfered portion.
However, in a method in which a lapping step is carried out after a chamfering step, there are some problems, that is, because the shape in plan or in section of the chamfered portion is got out of shape, and there is no opportunity to compensate the deformation of the shape, the deformed wafer as it is, is sent to a step for manufacturing semiconductor devices. Consequently, there is a problem that resist material to be removed remains on the peripheral portion of the wafer, in a photo-lithography step, and such a problem has contributed to the prevention of higher integration of semiconductor devices which will be required more and more in the future.
On the other hand, in a conventional etching step which was carried out just after a lapping step, the so-called acid etching, i.e., an etching in which the wafer was immersed in a liquid mixture of hydrofluoric acid, nitric acid and acetic acid, or the like, was carried out. However, because acid etching has defects that it is difficult to keep the flatness of the wafer after lapping and that high costs are required for processing the waste of the used etching liquid, recently, alkali etching, i.e., an etching in which the wafer was immersed in a liquid of sodium hydroxide, a liquid of potassium hydroxide or the like, has been mostly used, instead of acid etching. Because alkali etching is anistropic and is different from acid etching which is isotropic, when alkali etching is utilized, in particular, the rear surface or the periphery of the wafer is roughened, so that the smoothness of the wafer is degraded. Therefore, a further processing for the rear surface or the chamfered portion of the wafer is required. Particularly, there is a problem that the processing for the latter chamfered portion after alkali etching requires much time for processing the surface to have a roughness less than a predetermined value, to obtain the target smoothness, which is several times that of acid etching. When an alkali etching is utilized, in the step of polishing the surfaces of the wafer, a method to improve the smoothness of the rear surface of the wafer, in which a wafer is set on a carrier and the front and rear surfaces of the wafer are simultaneously polished by buffs which are set on lapping plates arranged in upper and lower sides of the wafer, is also carried out. However, in such a polishing for the front and rear surfaces of the wafer, some problems, that is, the chamfered portion of the wafer is shaved by the inner wall of the carrier to make the sectional shape of the chamfered portion get out of shape, and thereby in a photo-lithography step in the following manufacturing process of semiconductor devices, resist material to be removed remains on the peripheral portion of the wafer, may be encountered. Therefore, these problems have contributed to the prevention of higher integration of semiconductor devices.